The present invention relates to a novel ultraviolet-curable silicone-based releasing composition or, more particularly, to a silicone-based composition capable of giving, by irradiation with ultraviolet light, a cured coating film exhibiting excellent releasability from sticky adhesive surface to be suitable as a coating agent of the back surface of pressure-sensitive adhesive tapes, releasable protective backing sheet on pressure-sensitive adhesive labels and the like.
While several types of compositions are known in the prior art of the ultraviolet-curable silicone-based releasing compositions, the silicone compounds as the principal ingredient in most of these prior art releasing compositions have a molecular structure of an organopolysiloxane of which radical-reactive functional groups are introduced into the polysiloxane main chain. When such an organopolysiloxane is irradiated with ultraviolet light, free radicals generated from the functional groups promote the crosslinking reaction of the composition by the mechanism of a radical reaction. As an inherency of radical reactions in general, the crosslinking reaction proceeds very rapidly so that curing of the releasing layer can be completed within a short time to give very high productivity of the coating process for the production of pressure-sensitive adhesive tapes, releasable backing sheets of pressure-sensitive adhesive labels and the like to exhibit excellent releasing effect from a sticky adhesive surface.
In conducting curing of an ultraviolet-curable silicone-based composition of the above mentioned type by an irradiation treatment with ultraviolet light, one of the serious problems is that the oxygen molecules in the atmospheric air exhibit an inhibiting effect on the radical reaction sometimes resulting in incomplete curing of the releasing coating film. Although this problem can of course be solved by flashing the chamber for the ultraviolet irradiation with nitrogen gas to exclude the atmospheric oxygen as completely as possible, such a measure is economically not feasible due to the costs for the nitrogen gas consumed in large volumes and the requirement for an expensive air-tight sealing mechanism of the chamber for the ultraviolet irradiation treatment. Accordingly, it is eagerly desired to develop a novel ultraviolet-curable silicone-based releasing composition which is less susceptible to the inhibiting effect of the atmospheric oxygen against the curing reaction so as to enable the ultraviolet irradiation treatment to be carried out even under a relatively high partial pressure of oxygen with a great cost reduction.